1. Field of the Invention
This invention relates in general to certain new and useful improvements in fluid composition detectors and method therefor and more particularly, to gaseous fluid detectors and methods which utilize selectively semi-permeable membranes to permit differential permeation of fluid across the membrane and which permit continuous monitoring of fluid concentration.
2. Brief Description of the Prior Art
The leakage of fossile fuel gases such as natural gas, primarily methane, from gas meters and gas pipes results in considerable expense to utility companies providing natural gas and to companies piping natural gas and other gaseous products. This considerable expense arises not only from the costs of the gas which is lost through, e.g. pipe and other conduit and equipment, but the necessary repair and replacement costs, false alarms and the like. In addition to the foregoing, sufficiently large gas leaks present other undesirable problems such as safety and health hazards.
It has long been recognized that the development of a relatively low cost gas detector would be highly desirable both as a safety device and as a means for preventing the loss of such gases. Natural gas companies and energy providing utility companies which provide natural gas often use various forms of gas leak detectors. These detectors are generally quite expensive and usually must be constantly adjusted even when routinely examining for gas leaks. Due to the complexity thereof and the costs of manufacture, they are not readily adaptable for home use or use in installations where costs are an important economic consideration.
The typical customer or user of a natural gas product has little or no ability to detect a gas leak, particularly when the gas is methane and is normally odorless and invisible to the naked eye. Usually, in the case of a natural gas, the utility companies add an odoriferous agent, as for example, a mercaptan, to the gas in order to enable the user to smell the presence of the methane. However, even when an odoriferous agent has been added to the gas, the customer or other user of the natural gas typically is not able to smell the same until substantial quantities of the gas have accumulated. At this point, a substantial health and safety hazard already exists.
In addition to the foregoing, many people are not capable of detecting gases, even when containing an odoriferous agent. Further, in many cases, parties working with natural gas, as for example, at transmission stations and power stations, become acustomed to and thereby immune from the smell of small amounts of the gas. Thus, even when the gas has accumulated in substantial quantities, they are incapable of detecting the presence of the gas by smell.
Consequently, a rugged and simple to use, reliable and inexpensive detector of gas, such as methane, and the like for customer use, as well as for use by utility companies and suppliers of gaseous products, would substantially reduce many of the present problems. For example, such a detector would significantly reduce safety and related problems, substantially lower the incidents of false alarms as well as providing more economical transmission and use by the operating companies and the customers.
It has also been recognized that relatively low cost detectors of this type could be used in mining environments, such as in coal mines. In this way, it would be possible to detect the presence of ignitable gases which often accumulate in mines and create very dangerous working conditions. Here again, such a detector must be relatively inexpensive and rugged and highly reliable in its operation. Further, any such gas detector which is reliable and simple to use should be portable so as to be capable of being carried in a pocket of a user. In addition, any such device should have a high degree of sensitivity and must be capable of being easily calibrated. This is particularly true where the gas detector would be used in commercial and industrial environments.
There are presently available flame ionization gas detectors. While these detectors are effective since they have a high degree of sensitivity, in at least parts per million, the cost is quite substantial and may be about two thousand dollars for each such detector. Moreover, these detectors are not necessarily rugged and highly reliable, particularly in industrial and commercial applications. Furthermore, the cost of purchase and maintainence of such units is prohibitive against wide spread use of these detectors.
Recently, there has been provided a gas detector which uses a sintered N-type semiconductor pellet which is essentially comprised of tin oxide and doped with various impurities. A heating coil is embedded at each end of the semiconductor pellet. When a current is passed through one of the coils to heat the pellet and control the temperature thereof, the other coil will serve as an electrical contact for monitoring the electrical conductance of the pellet from coil to coil. Air is absorbed at the surface of the pellet when the sensor is exposed to air and if the air contains a combustible gas, the pellet will react with that gas. The reaction product is absorbed which, in turn, increases the electrical conductance between the two coils. In this way a change in electrical conductance will occur in the pellet when a selected gas is present. While these sintered semiconductor pellet gas detectors may be effective for home use, they are not effective for industrial and commercial purposes. These detectors have a non-linear output which is particularly flat in a range of about 40% to about 60% methane. Further, they are quite difficult to calibrate and consume a substantial amount of power.
There have also been various gas detecting apparatus which utilize a principle of differential diffusion through a porous wall of a vessel in an environment where the vessel may be surrounded by a gas or a mixture of gases which are to be detected. Each of these prior art devices are relatively inefficient either because they are not very responsive, not very accurate or they are too costly to manufacture. More importantly, each of these prior art devices utilize a pressure differential measurement which is inherently inaccurate and do not lend themselves to constant monitoring.
U.S. Pat. No. 4,122,736 to Wheldon et al discloses a device for detecting the presence of a gas contained in a mixture by use of a membrane which is preferentially permeable to that particular gas and which also uses a thin flexible diaphragm. However, the device is constructed so that it is pressure responsive, that is, a pressure differential is created when gas crosses the membrane into a particular chamber. The Wheldon et al device suggests that the flexible diaphragm has a substantial amount of structural integrity. This device, by its very nature and by use of the diaphragm employed, considerably reduces the overall sensitivity and effectiveness of the device and is incapable of continuous monitoring.
U.S. Pat. No. 3,438,241 to McKinley, Jr. also discloses a gas detection system which utilizes a membrane which is selectively permeable to a component of a gas. A carrier gas sweeps the component to a detector and measuring device such as a low volume katharometer.
U.S. Pat. No. 1,016,305 to Turquand et al also discloses a method for using a porous material for an alleged diffusion of gas through the material. The material described in the Turquand et al Patent is porous and the gas will permeate through the porous membrane. The membrane in the Turquand apparatus and method is thus like most prior art apparatus and methods, that is porous, as opposed to being permeable or semi-permeable. Notwithstanding, the Turquand et al Patent also utilizes a pressure differential for measurement of any gas diffusion.
U.S. Pat. No. 3,871,228 to Weiss et al discloses a saturometer which measures a total dissolved gas in a body of water such as a river or a lake, or the like. The Weiss et al patent uses thin wall gas permeable membrane, such as dimethyl silicon tubing. In addition, the Weiss et al patent also relies upon a pressure differential measurement as opposed to a voltage change measurement at constant pressure.
Other patents which relate to the use of differential passage measurement of a gaseous constitutent for purposes of measuring the presence of the gaseous constitutent include U.S. Pat. No. 2,811,037 to Beard, U.S. Pat. No. 3,546,922 to Dreckmann, U.S. Pat. No. 2,045,379 to Bennett, U.S. Pat. No. 1,746,425 to Heckert and U.S. Pat. No. 2,561,414 to Potts, Jr.
Other devices have been known in the prior art which utilize differential diffusion. One such device is taught in U.S. Pat. No. 1,174,370 to Webster. These devices are relatively inefficient and often totally ineffective. The device in the Webster patent, for example, uses a porous vessel in order to permit the infiltration of the fluid to be detected into the interior of the vessel. Devices utilizing differential diffusion are relatively ineffective due to the inaccuracy of measurement and relatively slow response time.
In each of these prior art devices, there is no means to compensate for external temperature and pressure changes as well as compensating for other constituents contained in the ambient atmosphere which may pass through the porous walls of the detecting chamber. This represents a disadvantage in that there is no means to compare against a standard for absolute measurement.
Many of the prior art devices, as for example, that taught in the aforesaid Turquand U.S. Pat. No. 1,016,305, utilize a porous membrane, as opposed to a permeable for semi-permeable membrane. The use of a porous membrane represents a disadvantage in that the diffusion rates across the membrane are much closer together than those obtained by using a semi-permeable membrane. In addition, prior art devices demonstrate a lack of selectivity, that is, they are not capable of providing a selective diffusion or permeation rate so as to be capable of acurately measuring even small constitutent gas changes.
In some of the aforesaid prior art systems for measuring the constituent change in gases, the devices are constructed so that they must necessarily rely upon a pressure change. These prior art devices have typically a slow response time as a result of their large chamber volume with respect to the membrane or porous surface area. They need the large volume to operate their pressure responsive indicator. Consequently, in these prior art devices, not only is the response time slow, but there is always some volume change which interfers with any diffusion or permeating reading. The pressure responsive nature proved to be a disadvantage in these prior art systems in that they were incapable of measuring slow changes. The prior art systems were also essentially insensitive to slow environmental changes.
In addition, and due to the fact some of these prior art systems utilized fairly thick membranes and particularly porous membranes, only a low level signal was achieved, which resulted in some inaccuracy. The fact that all such prior art systems made pressure measurements as opposed to volume change measurements at essentially constant pressure precluded any form of continuous monitoring inasmuch as it was always necessary to reset each such device back to a zero reading before a new reading could be made. In addition, and in many cases, it was necessary to purge the chamber of the prior art systems in order to obtain a true zero reading level before any new measurement or detection was made.
There are also known in the prior art, several capacitive pressure sensors, as for example, those disclosed in U.S. Pat. No. 4,358,814 to Lee et al, the displacement convertor disclosed in U.S. Pat. No. 4,357,834 to Kimura and the pressure-electric transducer disclosed in U.S. Pat. No. 4,096,758 to Moore.